You should consider the Fel-Pro VS50145R valve cover gasket set. These are constructed of a rubber-coated high-temperature fiber and are designed specifically for the 6-bolt BB heads, in particular for extreme heat applications such as trucks and emergency vehicles. The same gaskets are packaged in the Fel-Pro Performance Set #1612.
Application of RTV silicone is NEVER recommended by any gasket mfgr onto the gasket, excpet where there would be corner intersecting joints.
If the cast iron raised exhaust mflds are still used, the RTV will melt due to the close proximity of the heat/m,fld to the cover. Again, that is why these hi-temp rubber-fiber gaskets were specifically introduced.
No gasket is better than its installation. the #1 cause for valve cover gasket oil leakage is due to out-of-flatness of the cover's flange surface, as well as over-tightening of the cover's hold-down screws.
Trust me.